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A novel estimation for the critical size of the frontal process zone of ceramics is proposed using a single-edge V-notched beam (SEVNB) technique. A three-point flexure test is carried out on aluminum titanate ceramics containing a sharp V-shaped notch with different depth. An exact solution of the critical local stress is analyzed at a critical distance from the notch tip. The critical frontal process zone size is determined as the distance between the notch tip and the point where the
critical local stress equals the flexural strength of specimens without notches, based on the local fracture criterion and the Griffith-Irwin criterion. The critical size of the frontal process zone, the fracture toughness and the flexural strength were also estimated for several materials, such as, alumina, porous alumina, and alumina-based nanocomposites. The relationship between these mechanical properties indicated that there was an almost linear relationship between the fracture
toughness and the resultant of strength and square root of the critical frontal process zone size, and that both of them must be increased to improve the fracture toughness of ceramics.

Abstract: Multi-crack problems are deeply involved in rock-like material and rock engineering. In order to study the influences of lateral stress and inclined crack angle on the failure load of the multi-cracked body, uniaxial and biaxial compression fracture tests are conducted on plate specimens with regular distributed multi-cracks. The stress distribution and the stress intensity factors KI and KII for every crack tips of the specimens are calculated by FEM. The experiment revealed that the failure load of the multi-cracked specimens increase obviously with the increase of the lateral pressure σ2 and the inclined crack angle α. And the multi-cracked specimens will hardly initiate propagation under equal biaxial compression. Analyses have been shown that these can be explained by the variation of KII with the σ2 and the α.

Abstract: A two-parameter constraint-based fracture mechanics approach is used to explain the effect of the constraint on the apparently anomalous behavior of short fatigue cracks. The different levels of stress constraint are quantified by the T-stress, and microstructurally as well as mechanically short cracks are discussed. Short cracks generally behave more sensitively to the constraint than the long ones. It is shown that in most cases, the existence of short cracks goes hand in hand with an intrinsic loss of the constraint, which contributes to a decrease of their fatigue
threshold values and accelerates their growth. In this paper, the above effect is quantified and conclusions concerning the applicability of the fracture mechanics parameters and approaches to the estimation of the residual fatigue life of structures are discussed.

Abstract: Static and dynamic ductile crack propagation tests were carried out using thin single edge
notched tension (SENT) specimens of carbon-manganese steel, each of which had a fatigue pre-crack
or a sharp V-notch as a crack initiator. The crack tip opening angle (CTOA) was measured using
digital images on the surface of the SENT specimens, and the critical values of CTOA for crack
propagation decreased with increasing crack length while initial crack growth was still small. After
the initial crack growth up to the distance of the specimen thickness, the critical CTOA remained
almost constant. These tendencies were common in static and dynamic crack propagation specimens
as well as fatigue pre-cracked and sharp V-notched specimens. There was no particular difference in
the static crack propagation characteristics of both fatigue pre-cracked and sharp V-notched
specimens. On average, it was observed that higher crack speeds affected the constant values of the
critical CTOA by slightly reducing them. The constant CTOA tends to decrease with an increasing
global constraint factor, and this suggests that the factor is insensitive to a crack starter, fatigue
pre-crack or a sharp V-notch, but relatively sensitive to crack speed.

Abstract: The wedge splitting (WS) test is now a promising method to perform stable fracture mechanics tests on concrete-like quasi brittle materials. Fracture parameters, such as fracture toughness and critical crack opening displacement and et.al, are however not easy to determined since formulae available from stress intensity factor manual are restricted to standard specimen geometry. The paper attempts to compute expressions for commonly used fracture parameters for a general wedge splitting specimen. By means of finite element analysis program, test simulation was performed on non-standard wedge splitting specimen with different depth and initiation crack length, and thereafter expressions were proposed for stress intensity factor at the pre-cast tip and crack mouth opening displacement on the load line. Based on the work above, size effect on the unstable fracture toughness and crack extension were investigated, and the consistency of fracture toughness data for various specimen depth as well as initiation crack length is demonstrated. The crack extension is little sensitive to the initiation crack length, it increases with the depth of specimen, which can be explained by the boundary influence of the specimen.

Abstract: Crack growth path was investigated experimentally, numerically and theoretically using two test specimens subjected to pure mode II loading. The specimens were (a) the center cracked circular disc (CCCD) specimen subjected to diametral compression often called the Brazilian disc and (b) the diagonally loaded square plate (DLSP) specimen containing inclined center crack and subjected to pin loading. A few CCCD and DLSP specimens made of two brittle materials (i.e. marble rock and PMMA) were tested under pure mode II conditions. It was observed that the fracture initiation directions and the fracture paths for the tested specimens differed significantly and grew in two different trajectories. However, it was shown that the experimentally observed fracture paths for both specimens can be predicted theoretically very well by using the incremental crack growth method. Several finite element analyses were performed to simulate the whole fracture trajectories of the tested CCCD and DLSP specimens. At each increment, the direction of fracture initiation for the tip of growing crack was determined using the fracture parameters (i.e. stress intensity factors and T-stress) based on the modified maximum tangential stress (MMTS) criterion. The main difference in the fracture trajectory was found to be related to the magnitude and sign of the fracture parameters (which depend strongly on the specimen geometry and loading configuration) and also the type of tensile or compressive loading in the CCCD and DLSP samples.